1. Field of the Invention
The present invention relates to a vehicle brake apparatus, and more particularly, to a vehicle brake apparatus that distributes braking force to a plurality of wheels during braking.
2. Related Art
In a conventional vehicle antiskid, or antilock, braking system, the braking force applied to respective wheels of a vehicle is regulated so that the front wheels lock first to ensure stability of the vehicle body during braking. For example, front-wheel and rear-wheel braking force is ordinarily distributed so that front-wheel braking force transversely intersects a front-wheel lock-limit line without the rear-wheel braking force intersecting a rear-wheel lock-limit line, as indicated by the solid line in the graph shown in FIG. 10.
Consequently, during antiskid control, brake-fluid pressure of several wheels is regulated based on a road-surface friction limit (also termed a lock limit: a friction limit of the wheel and road surface whereat greater friction force cannot be obtained). By regulating the brake-fluid pressure in this manner, braking is optimally controlled when a road-surface friction limit is reached, or when the road-surface friction limit is approached in accompaniment with a braking operation performed by a driver, by initially braking the front wheels under antiskid control.
However, after the front wheels have entered antiskid control, the driver must depress the brake pedal until rear-wheel braking force reaches a point Em at which maximum braking force of the rear wheels is substantially demonstrated. For this reason, as shown in FIG. 11, not only considerable pedal-depression force becomes necessary, but also deceleration G does not increase in proportion to the depression force. Deceleration G is gradually increased due to an increase in rear-wheel braking force, and large deceleration G is rarely obtained. As a result, a relatively long time is required until a maximum braking force is reached for all wheels, thereby resulting in less-than-optimal braking performance.